Modulation



July 24, 1928.

1,678,163 E. PETERSON MODULATION Filed Dec. 29, 1923 2 Sheets-Sheet lf/g/ ffy;

Ec 2 @A y /m/e/f/ar:

' [age/7e fe/effo/r I July 24, 928, I 1,678,163

' E. PETERSON MODULATION Filed Deo. 29, 192s 2 sheets-sheet 2 smEBANn/m/efifor: [ae/7e Pe/erJo/y W Y Any Patented July i24,l 1928 UNITEDEUGENE PETERSON, or NEW YORK, N. Y., AssmNoR To 'WESTERN ELEoTRIc comm,INCORPORATED, or 'NEW YORK, N. `ir., n ooRPoRA'rioN or Naw YoRx.

' .iaoii-u.i:a'1irc JN. Application led December 29, 1923. Serial No.683,801.

This invention relates to modulation, and components or side bandSfaredistinguished particularly tovmodulation of higher order from each otherby the number of times the than the second, or other niethodsofmoducarrier and modulating frequencies, takenlation producing side bandsof similar fr e-` together, occur. Thisv distinction 1s made quencycharacteristics and its application in use of in defining the order`ofgmodulation 60 carrier wave communication systems. which producesthese componen s. For ex- Modulation, as used herein, relatesbnQadample, second order modulation produces' ly to that class ofphenomena which acsldebands of theI usual kind, -1n which the" companiesor, in fact, is produced by distor- Carller and modulating frequencleseach ootion of electric waves, and especially to llIS 0I1ce,th1rdorder.of modulation results 165,. ,phenomena which occur when two waves,one 1n Side bands in which the iirst even the so-called carrier wave andthe other the momo 0f either the carrier or modulating modulating wave,are jointly impressed ona frequency OCCIIIS, elle. f distortinginstrumentality, Anobgect of the in vention is to provide l5 It is wellknown that modulation of this mod ulalng-means Which-are simpler, mOre70"' kind yields side frequencies made up of variiC1eI1t, rugged, `andless expensive, than ous combinations involving thefrequenciesmOdllla-tlllg Ineens heretofore. invented.` of the impressed waves. Whenone of the AIlQlJheI ObJeC? 1S the ICtlCal lltillzatlll waves, eitherthe modulating'or carrier wave 0f thll'd, and hlglll 0I' er, mdulfltlon,0r v 2" consists ofaband of frequencies the side fre 0f modulation sidebands of similar fre- 'Ni quencies likewise assume the form of bands.'quenCX- Charactellstls- The number, amplitud@ and frequency A stlllfurther object is toproduce a s u characteristics of these side bands isa vunc- Pressed CaI`1`191, S1g11&1mofhlkttedwave, Wit tion of the kindand extent of the distortion out the US@ Ofi balllcd CHQUIS .g5 asmeasured, for example by the equation Another obJect isto effect odd-orderinodu- 80 of the characteristic curve which expresses 135mm 't0the Slbstantl-QXCIUS-l-Ol 0fv ,even the relation between the amplitudeof the Order n lodulalom input and output quantities. 'A 513111. further0bJeCt;1$ -'t0 Pr0V1de 1,1m- In the present systems. use is made of theproved means f Ol' emratlng thildlderslde i square characteristic ofthis curve, that'is, bands o f relative y greatamplitude.' 85. thecharacteristic which makes the equation `Add-11-0na1 Oblecs 0ftlle'lllvelltlOn are: I ofthe curve approach the form i/=c2, in t9aChlVBhQmJmC multlplex 009111111111125- which a: and ,g/ arerespectively the 'ampli- UCP, 15mg a' .Slngl `S0UFC9 0f Carmel. WaVeSftudes of the input and output quantities. Pllme 0 1' Seconflal'yff 11 0fthe Channels; Other terms having other powers of w may, t0 PlOVlfe @Il@IDPI'PVB Sgsfem 0f WO-Way 90. to some extent/,be present. Those inother CQmmPlllCatlOIl h YlDg d1 eljellf C alllfld" even powers of willcontribute to the effect quencle and a Single Cal''le. SUICBifthGHsecured by the square term as will be ex- P fodllctloll )f OddhPmOmQSfor u Seas Carplained later. On account of this square nel'ffeqenclesiand t0 PIPV1C-1@ im. Improved .40 characteristic, modulationyields side bandA System 0f Sgecret pnlmulllcatlo 95 components havingfrequencies equal respec- The Val'lOllS means .Suggested for pIO- tivelyto the sum and difference of the 'f re: ducing pronounced' higher Aordermodulaquencies of the modulating and carrier tion side bands eachinvolves means'for prowaves. .These side. bands with or Withoi1`t\diici'ng', andl operating over, a'characteristic some of the original-carrier constitutev a, A cnrvekwhich has one or more pronounced 190modulated carrier wave from which the bends. Ini-the particular case -in.which it y I mOdlllatlPg C OIHPQHGH may he reproduced is desired thatodd order modulation side f at 'a receiving station by a similardistortion. bands should be obtained to the-exclusion 0f The termsbesides the square term, if the even order side bands, the .curve should5o present, in addition to contributing to the have -two symmetricalbends ,in relatively 105 effect 0f the sql1al`e elm', @S abOVe, 111:0-opposite directions, that is the curve-should duce other side bands in.`which lharinonlcs rhave, 0 positesymmetry about each of the of thecarrier or modulating' frequencies, or axes o? coordinates Vthrough aoint on theY both, insteadof the fundamental frequencies, curve and theoperation s ould occur occur. These various combination frequencyequally over -thej curve in both directions u about the point. If theoperation does not occur about the point of symmetry or ,if the curvelacks symmetry of this kind, even order side bands will be present.Simil larly, even order side bands can be obtained,

tothe exclusion of odd order side bands by operating over a curve whichhas opposite symmetry with respect to only oney of the axes, as, forexample, a parabola. These two forms of symmetry are often, and moresimply, designated respectively symmetry about a point and symmetr abouta line In general, for either case, t e acuteness of the bends indicatesproportionall the order ltothe substantial exclusion of even order sidebands. -The lmodulating and carrier currents are used to variablymagnetize the core by impressing them on a common coil, or on differentcoils, mounted on thecore. On accountof `its unusual magnetic qualitiesthe alloy known as permalloy has been found to be ideal for the purpose.This alloy is described in a paper by Arnold and Elmen in the Journal ofthe Franklin Institute for May, 1923.

lAl similar effect can be obtained, using electron dischargerdevices byinserting a high .impedance in series with the grid of a conventionalthree-electrode discharge modulator. The impedance may be a conductiveelement of the usual kind or a'halfwave rectifier. An abrupt bend in thenormal space current grid voltage characteristic curve occurs at thepoint corresponding to the instant when .an increasing im ressedalternating'current voltage passes t rough zero value, on accountv ofthe rrelatively large voltage vrdrop that occurs in the series impedanceat and subsequent to this linstant of time. According to the criterion.mentioned above, pronounced even order modulation effects can beobtainedfrom altube having this characteristic. By operating beyond thefoot of the normal characteristic,

i a second abrupt bend in the opposite .direcfrom a modulator havingthis c aracteristic. Somewhat similar effects canv be obtained" tion canbe made to occur at this point. Pronounced odd Vorder modulation eifectscan be secured, as has been ointed out,

by the use of an'arrangement consistingof be secured by the use ofoneisuch combination.

The symmetry obtained by using a push.

be utilized to produce odd order modulation Y side bands to theexclusion'of even order side bands, oi vice versa, with accompanyingsuppression of the impressed carrier orV certain of its derivatives. Theuse of this general arrangement or another arrangement including avsingle tube circuit having the requisite symmetry of characteristicwith an out ut filter, makes possible theproduction ci) higher orderside bands with complete suppression yof. the components which couldotherwise be used inv combination'with the received side bands at adistant station, b even or odd order deinodulation, to repro uce thesignal.' In particular a single tube or a magnetic core modulator whenlarranged to have syinmetr i about a point suppresses, for example, t edouble carrier. Since the .fundamental carrier may easily be separatedby a filter from the resultant third orderv side bands the use of amodulator of this type can accomplish a result similar to thataccomplished by -a vbalanced arrangement of tubes in second ordermodulation with carrier suppression.'

Systems using higher order modulation side bands of similar frequencycharacteristics may use the same order of demodula-A tion l,if theoriginal carrier is transmitted' or,

'locally vintroduced at the receiver. If a and demodulation may bedifferent.

If a lower third order side band vor a side band of `similar frequencycharacter-y istics produced by other orders of modulaor other methods ofmodulation producing tion and the im ressed carrier are transmitted, andthe ouble frequency carrieris suppressed, and if the frequency of thecarrier is chosen close to the upper limit of the impressed signal band,secret telephone communieation is possible since an interceptor providedonly with second order demodula-l tion circuits will hear invertedspeech. 'A receiver provided with a third order demodulator whichsuppresses the even order side.'

bands would be able to reproduce speech to the exclusion of theiinvertedspeech. l

Harmonic multiplex communication is made possible by using one carriersourceI and different orders of modulation for the -f Levanta 3 Theinvention both as to its theoretical principles and its practicalembodiments will be better understood by reference to the followingdetailed description, together with the accompanying drawing, in which,

Fig. l is a graphical diagram illustrating certain theoreticalprinciples of the invention. i v

Fig. 2 discloses a circuit for obtaining' odd order modulation withcomplete carrier suppression.

Fig. 3 discloses-a circuit for obtaining pronounced odd order modulationside bands with elimination of even order side bands and even powercarrier harmonics, or conversely for obtaining pronounced even ordermodulation side bands with elimination of odd order side bands and oddpowerl harmonics.

Figs. 4, 5, 6 and 6" are graphical diagrams illustrating certaintheoreticalprinciples of the operation of the systems of Figs. 3 and 7,.

Fig. 7 discloses a modulator which may be used alternatively to thatof-F ig. 3, to acF complish substantially the same result.

Fig. 8 discloses a typical one-'Way carrier current system employinghigher order modulation and also one form of magnetic core modulator.

Fig. 9 illustrates graphically certain thei `oret-ical principlesinvolved in the operation embodied .and to properly appraise its ad-llvantages as applied therein, the following' of the modulator of Fign-8.

Figs. 10, 11, 12, 13 and 14 disclose other orins of magnetic coremodulator, and ligs.A 15 and 16 disclose systems alternative to that ofF ig. '8 for utilizing the principle of modulation of higher-order thanthe second or of methods of modulation producing side bands of similarfrequency characteristics.

In order to aid in an understanding of the general nature oftheinvention and to elucidate the characteristic ualities ofthe variousspecific systems'in w ich it maybe mathematical analysis of modulation,especially higher order modulation, is given.

If currents of different frequencies p and g are fed into 'a distorting(modulating) device', there results a complex currentv in which thefrequencies have the general form mpi-ng, in which --m and 'n may. haveany or all integral values, or zero values, and in which the symbol:indicates that the sum,

' the difference,- or both the sum and differ-A uantities may bepresent. This statement will. be proved later when ence, of the two thecomplete equation expressing the modulated current is derived.

When either of the coefiicients 'm or n has zero value, the currentdefinedA by this expression is a direct current if the other coefficientis zero. If the other coeficient is instead of side frequency.

not zero, the current will have components the frequency of which is por g, as the case may be, or various harmonics thereof. When neither ofthe coefficients has zero value there are obtained combination frequencycomponents, that is, side bands. The order of modulation is convenientlygiven as the sum of m and lVhen both m and nare unity wel have thefamiliar case of second order modulation in which the side frequenciesare pi -75 Third .order modulation may be correspon ingly represented by21otg or 32129. There are accordingly four possible third order sidefrequencies. If r is a number expressing the order of modulation (thatis, a number which is the sum of two numbers) it is evident that thereare r-1 combinations'of different numbers' which add up to equal 7,-

so that, considering bot-h the sum and difference frequencies, there mabe a maxi- 85 mum of 2 (1f-1) different si e frequencies for each of therespective orders of modulation.

In what follows, it will be assumed that g represents a band offrequencies as, for example, a voice current (the so-called modulatingcurrent) and that p represents a fixed carrier frequency having a valuegreater than the greatestvalue of y. The expres-4 sion side band willaccordingly be used 95 Also, only the side bands in which occurs once,that is, thosehaving frequencies pig, 23919, Bpiq, etc., will beconsldered, since obviousl onl by the transmission of side bands of thistype 10o can the modulated component be reproduced" l at the receiver bymodulatinfr such lside bands with a wave having the frequency of the.unmodulated carrier or a harmonic theremodulation is obtained bysubstituting inthe general equation 'of the type y= am 6x2 -lom?l 115v`values of simultaneously impressed cur` rents (or potentials).,Sup'posefthat the 4input currents' are? co'spit and -Q cos glt,

in which j), and g, equal respectively- 2n-y and 277g: cos gli?. (-Nomaterial change would result if an initial phase angle between'the twoimpressed Waves were assumed.) l This value of w should be substitutedin the general equation.- The first term a w 125 yields merelyamplifiedwaves of the impressed frequenciesv p and g. The term 6x2 yields wavesof' frequencies 2p, 2q, and

pig, as iswell known. The second order l slde bands .pig result from thetrigono- 139 A metric expansion of the vproduct bPQ cos I the binomialtheorem,'it will be ound' that pj cos glt. and 2g result fromtrigonometric expansions of, respectively, IDP2 cos2 y pla and V bQ2cos2 glt. f y

If each of the remaining termsof the eneral equation'are algebraicallyex ande by the expanded equation comprises terms in powers of cos pltand cos gli, andv terms l which contain the productlcos plt cos g1t as afactor. The first two obviously yield waves whose frequencies' are,respectivel harmonics of p and g. The last obvious y,

yields combination frequency waves, that is,

' side bands.

Consi'deringfthese terms which produce c ombination- 4frequency waves,lit will be found that alternate terms, beginning with the second, in theexpansion of even power terms of the general equationare of this' typeand are further characterized in that the exponents ofcos p1t and cos g1t are each odd'. These terms in vtheir regular order, u tov andincluding ,the expansion of the sixt power term of the general equation,are as follows, only those-coefficients which are necvessary in thisdiscussion being retained:

The frequency determining quantities in these forms are each in the formcosine @1t cos gp5- or this quantity times one or more cosine squaredquantities. Since cosa=1/+1/ cos 2a the development of these quantitieseach contains one term of theform of 1/2 cos plt .cos glt. Thisdemonstrates that the terms of (l) each yields a pair of second orderside bands. These side ands for theseveral terms-are superposed tocomprise resultant side bands.

v The amplitude of each of rthe resultant upper and lower side bands mayaccordingly be expressed as a series the termsof which are proportionalto PQ, PBQ, PQ, PQ3,.PQ,

A f P'aQs-:all being products of even order.

These terms will he multipliediby the co# efiicients b, d, f, etc.,`ofthe general equation.I

In general these coeicients decrease in mag -nitude asV the power of theterms of the general equation increases,'that is,y asl the order of theabove products increases. 'the characteristic curve may be caused -to-be substantially square so that substantially-only the term PQ, whichis linear in Q, is present. For other cases the other termsv will bepresentV in relatively small amounts and will introduce somedistortionon account 'of nonlinearity of certain 'of the The harmonic frequencles2pcoefficients in Q. However, a number of the terms as PQ, PBQ, PQ,etc., hence their sum, will be linear inQ. The magnitude of thedistorting terms, that is, PQ3, PQ, PSQ,

etc., may be minimized by making P large as compared with Q, lso thatsubstantial linearity may be obtained, as is necessary fordistortionless transmission. This is true even though the characteristiccurve departs, widely from its square configuration.

It may be shown, in a manner similar to the above, that other side bandsof a diiferent eveuorder may be obtained from other-terms than thoseindicated in (1 in the expansion oftheeven power'terms of the general eua-l 'tion. However,-as will be more evi ent later, only those of theform mpi g will have coefficients which are linear in Q and, therefore,useful. n s

From the odd power terms of the general equation odd order sidebands-may be similarly obtained. This will be demonstrated for the caseof third order side bands.

From the expansion of these odd ower p terms (except the linear term) itwill be found that alternatev terms, beginning with the second, haveeven powers of cos4p,t and odd powers of cos glt. These terms 1n theirregularorder, up to and including those for the seventh powerterm of thegeneral -equation, are as follows, retaining, as in (1) only thenecessary coeiiicients;

`ment of each of the terms in (2) contains a lll) term of the form cos212,1? cos glt. -This form' is similar to the form cos pltcos q,t and 1nan analogous manner yields upper and lower side bands of 2p. This demonstrates that the terms inl (2) each denotes a In fact third. order sideband of the itype Qpig.`

The amplitudes of each of the vresultant upper and lower side bands mayaccord-- ingly be expressed as a series, the terms of which are proortional to PLQ, PQ, PQ, P2Q', PQs, 2 Q". The sum of these terms islinear under the conditions discussed above lfor second ordermodulation.

That is, when there is only the third power third order side bands ofthetype 'piQg may beobtained from othersterms than those indicated under(2), of the development of the odd power termsof the general equation.The quantities expressing the amplitude of these side bands is made uplof terms none of which are linear in Q, sopthat the sum is not linearand cannot be made linear in Q. This means that third order modulationof this type cannot yield a faithful reproductionv of the signal. Thesame thing is true of other higher orders of modulation, even or odd, inwhich n is greater than one. These side bands may, however',v

. be used in signaling where accurate reproduction ofv the modulatingwave is not essential.

Side bands of a higher odd order than third of the type 'mpg can also beob-A tained fro'mthe oddpower terms-of the general equation. `These sidebands, as well'as the higher even order side bands of the same type, canbe made substantially linear in Q.

Although telephone systems now in use depend upon second 'ordermodulation and demodulation, experience has shown that certain higherorders, especially the third, are substantially as suitable for theproduction of side bands and for the reproduction `of speech. Asillustrating the practicability of using the higher orders of modulationitwas-recentlyfound that in an actual carrier current telephone systemarranged for optimum second order modulation condican be demodu'lated toreproduce a signal by veither second or thirdorder demodulatiomtionsan'd for transmission of the unmodulated carrier component, theamplitude of the third order side bands could, by adjustments notaffecting the conditions of second order modulation, be made twice asgreat as l that of the second It is apparent that theterm carrier fre-vquency must fbe re-deined for use in describingA systems of modulationof higher Aorder than the second. For example, a third order side bandhaving frequencies Qpig,

depending uponwhether a current of frequency 2p' or p is available.Accordingly,`

depending on lthe order of demodulation,

" e'itherof these-two frequencies vmay play thel rle-playedl by thecarrier frequency in a system using second order modulation (andaccordingly second order demodulation). In

i this specification the terms carrier current" and ccarrier wave willbe applied to any current or wave thatmay be combined in a distortingdevice with the transmitted Side band or side bands to produce a signal,and the frequency of such current or wave will accordingly be designateda carrier ,frequency. A modulated Wave ofa high order may accordinglyhave ascarrier frequencies both the frequencies of the' impressed highfrequency wave and certain of its harmonics. The impressed highfrequency wave -will be designated as the impressed origi- Conditionsfavorable to the productionA Fig. 1 in which A andB ara-respectively,

the Ec-Ib characteristic curve and the curve of the second order sideband output current of a modulator recently tested. The modu-l latorcircuit was'of the general type illustrated in U. patent to Vander Bijl,No. 1,350,752, issued August 24, 1920. A tuing-k sten filament carryingacurrent of 1.35 amperes was used. The plate potential was 220 volts.VThe important thing to notice about this figure is that when the grid isgiven a polarizing potential of -18 volts, thev second order side bandbecomes substantially zero. This value of grid potential marks a pointof symmetry of the characteristic curve.

This condition, which is unusual and difficult to obtain with oxidecoated filaments, Was made possible byreason of the choice of filamentmaterial and the critical values of the constants used. The ,other `evenorder side bands would also be found to be substantially 4zero lifmodulation occurred about the same point.- Although the odd order sideband output is not shown in the curve, itis nota minimum at that point."

a point is. represented by a power Iseries,

this series will have no even. power terms,

fthat is, thel coeilicients b, d, f, etc.,lof the generalequationwill-each have zero values.

Although it is 'not necessary, inorder' to -produce odd order sidebands, to` operateA about a point of symmetry, such method of 4operation makes possibleodd order modulation with suppression ofeven'harmonics of 20 p the impressed carrier frequency'and 'withsuppression` ofeven order side bands since these components result fromthe termsin` lthe general equation having the vsame ex-l 'l lponents asthe numbers indicating thev frequency multiple. This expedient is ofvalue,

in its economy of energy, in the avoidance ofinterferencedue to thepresenceat'the receiver o feven orderl side bands, and,v on account of'its secrecy. `Such aniysystemis relatively secret, since it insl'lreltha Fig. 15 and which will be describedlater)l sup an Y' means of allilter.- l Since the odd. order si e-..

the 'expedient insures, a -vgreater se arationi between message carryingside ban I t f' Economy: #nd Sel'fci-al'e--Pmmotd inf"- odd' ordermodulation y suppressionfhat is, .byV Suppressinsth 'im' ressed carrleras well as its-even harmonics. ismay-be' accomplihed, for example, by

'.suppressing'fthe. impressed carrier b bands are 'far removedfrom theimpression carrier, separation" can easily be 'effected by ,the filter.-V Suppression of the carrierl frequencies may be accomplished' in thissimple4 manner without resort -to rbalanced' arrangements of tubes.AThis is of importancein instances where, ifthe usual second ordermodulating methods were to-v be used with carrier suppression, thecarrier frequency would be too high to rmit separation from the secondorder side ands'without resort to` such balanced arrangements.4 Y

Where lt-is diilcult to provide a single tube having the requisitesymmetry of characteristic to produce, exclusively, odd order Sidebands, a push-pull arrangement-l of modulator tubes may be used toproduce such an effect.v Fi 2 illustrates such an arrangement. In t isfigure, p and q .indicate, respectively, the sources ofthe correspondingimfpressed carrier and modulating currents. I

same direction, and if coils 3 and 4 .are

wound in'relatively opposite directions, asA

indicated, this arrangement of-.tubes will produce odd `order. lsidebands and will suppress the' modulated even order -side bands and oddcarrier frequency harmonics.

` Filter Fmay be used to sup ress the impressed carnerand its evenarmonics up to that harmonic'whichcorresponds to the particular'orderofmodulation used. For

example, if only third order .side bands 'were to be used the filterwould be desi yto vsuppress only the second harmonic o the impressedearner, if fifth orderV side bands 'were to be used the filter would supress' the Vsecond andl fourth harmonics. v ither of thesefconditionswould effectively correcomplete carrier su pression.

wind to s arrangement 4of circult, as we 1 as other balancedminuitarrangements to be described below, is' quite similar to certain'l forms of push-'pullmodulators' discussed in U. S. patent to CarsonNo. 1,343,306, issued June 15, 1920, which also contains a generalcomplete'. carrier ressmg the even harmonicsas above,.'

charge modulator, the

v device.

the ,tubes have identical char- -acteristics,if coils 1 and 2' are woundin the wel@ Alternatively',i`the two sources could be interehanged.;IThis arrangement would not v produce higher order side bands in thesense that this expression is used in this specification, lthatis,- o fthe fornijmgvL-g. The arrangement .therefore would not bel useful vin higherordermodulation.

Byr'ev'ersing either coil 3 or coill 4 in the arrangement of Fig. 2,even order vside bands ma 'similarly be produced.

or demodulation in which, depending o n the adjustment of .thequantities concerned,

pronounced even ...orodd .orderside bands. ma be obtained. l

ig..3 illustratesl a system of modulation ssuming for the present thatthe-switch which short .circuits the transformer 6. isl

closed so that this transformer is ineffective, circuit' 5 is connectedon one hand to the 'source of currents to be modulated or delmodulatedand on'the other hand through transformer 6 to devices 7 and 8. The

formerv may be' any type of' electric disatter a rectifierfor example, atwo-electrode electric discharge Reference ynumeral 9 indicates a grid'leak impedance for preventing too great an accumulation of negativecharge on the grid of tube 7;. This impedance a resistance in the'caseillustrated) is preferably high, for example of `the order of a megohm.The output currentl from; the combination may be used as'required incircuit 10.

The theory of the operation of the circuit lll() is best explained withreference to Figs, 4, 5

and' 6. Curve abc of Fi .`5 is characteristic curve of tube 7 plottebetween grid-filament potential and plate-filament current.

If the rectifier tube were not present so that the potentials would beimpressed directly between the rid andfilament of tube 7, the variationso impressed otentials would occur over the ortion bc o the curve 'intlfe usual way an sincethis portion of the curve exhibits ,onlyslightvariations of impedance, the form of the plate-v ilament currentwould be a substantialre- .production of the form of the impressedpotentials.

With the rectifier in circuit, the characteristic curve is modified, soas to assume the form bd, over the operating range. The platefilamentcurrent, plotted on a time axis will be as indicated in Fig. 4, if asine wave of impressed potentials is assumed. In this ligure, the timeperiods represented by intervals between 1, 2, 3, 4, etc are equal.

the non-horizontal portion of the characteristic curve were straight,the non-horizontal portion of the curve of. Fig. 4 would have a sineWave configuration. With the' characteristic disclosed, this portion ofthe curve nevadas- Ar p 1 i Aofi-tig. -4 is somewhat less pointed thanla" sine wave curve. -Accordingjto thej'criterion establishedabove, the`modification of the impedance varies by a very large factor, one

hundred in a known instance, when the polarity of the potentials.impressed are reversed. When positive half waves are impressed on thecombination of tubes, that is, when the grid of tube 7 is positive` withrespect to the anode-element-of tube 8, agridlament current tends toflow in tube 7 since i its grid is positive relative to its filament.The impedance of theinput circuit will correspondingly decrease. Thetotal potential-l will divide according to the respectivel impedances ofthis input circuit and of the rectifier. The result will be thatsubstantially al1 of the applied potential is impressed in eiect acrossthe rectifier. The potential impressed across the input of the tube.7will correspondingly be` very small or zero and the plate-lament currentwill uniformly, throughoutthe half cycle, have the value correspondingto zero grid. potential. When negative halfwaves are impressed on thecombination of tubes, the conditions are the converse. The rectifierwill have substantially zero impedance, the input circuit will havesubstantially infinite impedance, and the potentials will divideaccordingly. This condition corresponds to the normal condition ofoperation of modulator tubes with negative grid potential and theoperation will therefore occur over the normal portion b ofthecharacteristic. y

If the variations of potentials are sufficient to cause operation beyondthe foot of the characteristic curve so that the modified characteristicassumes the form e a o d, the conditions are favorable to the productionof odd order side bands. Fig. 6 indicates the correspondingplate-'iilaxncntcurrent curve plotted to a time basis. In this'curvetime periods indicated by intervals between I points l, 2, 3, 4, etc.,are equal, as in Fi 4.

It is.v apparent that'. the modified c aracteristic would be changedifthe maximum amplitude of the impressed potentialsl were changed, sincethis would result in the oper-- ation over a greater or less extent ofthe horizontal portions of curve e al o d or, otherwise stated, Iitwould result'in the inclined portion of the characteristic curvelbecoming relatively greater or smaller. The corresponding effect `onthe current-time curve would be to change the slope of the sides of thecurve. An increase in the maximum amplitudewould cause the curve4 toapproach a rectangular configuration. A rectangular configuration wouldbe characternent curves. Fig. 68,

iaedbyl symmetry between consecutive half t waves, since the timeperiods corresponding to the intervals between points 1. 2 3 4., etc.,and 1, 2, '3, 4, etc., would all be equal. This limiting condition,which corresponds to an infinite maximum impressed potential,

defines 'a situation in which there is an entire g absence ofeven-harmonics (if amodulating wave is superposed with the carrier waveon the -modulator, an entire absence of even order side bands).The'corresponding com pletesymmetry inthe characteristic curve`- whichshould define this condition, is that becomes relatively `zero ascompared with the infinitely extending horizontal portions.

-whichis attained when the inclined portion I so Byv using a moderatelylargel amplitude of.

carrier potential, this y limiting condition may be -quite .closelyapproximated.

A symmetry not dependent on the amplitudes of the impressed potentials,may be obtained by adding the other circuits of Fig.- 3 shown between 5und 10, as by opening'the switch which short circuits transformer 6.v

There is no mutual inductance between the two sets of transformerwindings 5. 6 and 6a so that the .openingy or closing of the switch doesnot aEect 'the operation of the transformer 6.` The prlmary andsecondary windings of transformera should be oppositelyrelated ascompared with the corresponding windings of transformer 6. This resultmay he accomplished by reversing lthe direction of winding of one of thecoils 'of transformer l 6l with respectto the corresponding coil oftransformer 6.

The `addition of the lower set of circuits'will result in a currentlindi` j i cated by curve ea/ b d of Fig. 5 in the lower half of theprimarytransformer during the time in which the current in the upperhalf is as indicated lby curve e a o d of the saine figure. The curve ofresultant current in the primary, and ac-l cordingly the form of thecurrentwave inl the secondary, of the output transformer, with referenceto thepotentials -a'p liedto the combined primary windings o? of theoutput 'i transformers 6 and 6a, is the sum of these curves.

n order to avoidunduly complicating the' figure this curve is omitted.-It is apparent, however,

straight about the origin andwould have i.

pronounced bends' at the ends, if the operaf tion extended that far',vcorresponding lto the bends at the feet of the respective compo-r Figs.4 and 6 illustrates the current .in the primary winding of the outputtransformer y.for this method of operation. Because of which correspondsto .115.- that .it would `be substantially' y windings of the primary ofthe out ut transformer are in aiding relation'. I they are oppositelywound, even order modulation side bands may be produced to the exclusionof odd order side bands and odd power carrier harmonics. p

A' result similar to that'obtainable by one pair of the sets of tubes ofFig'. 3 can be obtained by substituting for the rectifier of` Fig. 3, aresistance in the grid lead of a single three-electrode tube, suoli asresistance 11 of Fig. 7. If the potential variations across thesecondaryottransformer 6 (Fig.

7) causes the potential of the grid to tend toA become positive, thepotential drop in resistance l1 will modify this portion of thecharacteristic curve so as to cause an-abrupt las ' bands.

bend like that between b and al of Fig'. 5.

Although' electric discharge deviceshave so farbeen considered thenumber of kinds of devices that may vbe used for higher order modulationor deii'iodulation is limited only.

by the number of those capable of modulation generally. The method inwhich modulation is effected by varyingthe -inductance of a coil bycorrespondingly variably saturat-ing its magnetic core, isespeciallyrapplicable t0 suppressed carrier, odd order, modulation. Fig.8 illustrates one modulating arrangement of this kind and also a systemin which it, or any equivalent means, may be used.

In Fig. 8 numeral 12 indicates a modula-l tor of this type whichconsists of a toroidal core wound with a single coil. The use of thecondenser shown in dotted lines is alternative and will be referred tolater when considering Fig. 12. The function of modulator 12 is todistort the currents flowing therethrough in the mannerre uiredtoprodescribed in U. S. patent to Blackwell, No.

1,261.096, issued April 2, 1918.

Modulator 12 suppresses even-harmonics of the impressed carrier and evenorder s ide The modulated wave is impressed by transformer '16 on filterFB which selects thel side bands of the order desired or "a single sideband ot that order. The transmittedv currents are impressed on thetransmitting conductor 18 through three-winding` Y transformer 19.vDevice 17, which may be similar to 15, maybe usedl if desired, to

.regulate the amplitude of` the transmitted side band. -It may equallywell be used at the receiving station. 'Circuit 18 may be a transmissionline connecting two communicating stations or pair of radio antennae,

lone at the transmitting station andA another at the receiving. station.

Carrier current for demodulation is sup' plied throughv circuit 20, theswitch 123 being closed, to points in the transformer 19 which' arebalanced with respect to the modulated wave input circuit. 21 balancesthe line 18. The amount of carA- An artificial net-work n l l riercurrent energy required tor the par ticular modulating anddeniodulatiiigg; condition may be critically adjusted by device l22. Theuse of 'the three-Winding transf` the distant stations serve to receiveand uti-- transmitted thereto. Assume, for the instant, thatswitches 125and 126 are open.

ylVith the particular arrangement so far described, since the carrier istransmitted, deinodulator DM must be arranged for deinodulation of thesaine order as the order of modulation used at the transmitting station.ny one of the specific types of vInodulators or demodulatorsvdescribedin this specification or their equivalents in function may be used. i j

The signal component is separated vb low pass filter FL and impressed ontelep ones 23. this and other figures of the drawing may be of .any typecapable-of vthe functions indicated. U. S.- patent to Campbell, No.

llize the wave impressed on conductor 18 and rl'ilie filters shown inseveral places in 1,227,113, issued May 22, 1917, describes varionstypes of `filters' from which selection transmission characteristicdesired. i

The operation of the magnetic core modulator of Fig. 8 will be explainedby refer- .may be made 'according to the particular i ence to Fig. 9.The curvefand g in thisy figure are the familiar B-f tween themagnetizing force H and magnetic induction B. .Curve f is the -mag-lnetization curve for positive variations of` H which would be obtainedif a magnetic core, as, for example, the toroidal core in the magneticmodulator, having no initial netized by positive values of current. The

' residual magnetization, were variablyv mags.V

curve g/ would be similarly obtained if neg-- ativev values ofmagnetomotive force were used. Curves f and g, taken together, ex-

hibit perfect symmetry about the origin.

The permeability of the core is. measure the ratio of B and H, that is,by the slope- 'Y ofthe chords of the magnetization curve between theorigin and the referencepoints.

masias The curves which-express the absolute values of permeabilitycorresponding to'curves f and g have approximately the shaperespec-ytively of curves z, and z', which obviously anceof the magnetizingwinding, the ing ductance varies 1n accordance with these curves. If thevariable.saturation of* the toroidal core with variable positive landnegative values of -impressed potentials, is assumed to follow thesecurves, the characn teristic curve 'between the" impressed potentialsand. current -owing through" the coil would be obtained by dividingthese positive and negative values by theordinates of z and e'. y wouldcorrespondfor example, tothe characteristic curve of Fig. 5, and woulddeterY mine the modulating properties, of the cirn cuit.

The curve would obviously havevsymmetry o the same type as that of curvef g, that is, it will have symmetry. about a .point. Accordin ly, underthe assumed conditions odd vor er modulation would l be achieved withsuppression of the even order side bands and of even power harmonics ofthe impressed carrier'frequency.. This result 1s achieved wlthout theuse of a polarizing means.

As a practical matter, the variation of under conditions assumed aboveof variable a the symmetry ofthe loop insures that these permeabilitycurves are symmetrical.A Accordingly, the loperation of the magneticcore modulator, even when there is -h s!v teresis, has the desirablefeatures 'of odd order modulation with suppression of the even ordereiiects.

' Of course, the hysteresis loop disclosed .would result only if themagnetizlng current has not-.more than one maximum and minimum percycle, as for example, a sine wave current. In the actualcasethemagnetizingA current consists` of superposed carrier an modulatingcurrent, and the resulting wave lform accordingly has numerousirregularities and reversals of slope. For'this case the hysteresis loopdisclosed illustrates, for example, only the uniformly recurrent loopcorresponding tol the carrier current.y The irregularities due to themodulating current would properly be shown as both large and Theresultant characteristic curve l may be considered broadly as avariation of small hysteresis loops having their origins ilu the loopdisclosed.4 f

It is. desirable to-use a core whichjsaturates ata small value ofmagnetizing current so that the characteristic curve has pronouncedbends at its two extremes, and accordingly so that there arejmorepronounced high. order modulation. eiects. From anf other point of view,the use of; a saturated .core makes possible the-production of .highv AI `order effects with the ,smallv valuesotim'` must often be used. l v

Although in thel above the effect of varia'- tion of inductance has beendescribed, the corresponding variation of leffective resist. ancecontributes,"A although usuall to a smaller extent, tothe result, sothatt e eiect pressed' potential, 4that for y'practical reasons,

im edance.

n practice, 1t has aoy ssi"

the core markedly improves the operation of the device. This metal is analloy containing two y elements lof the magnetic group, such groupbeingv made vup'of manganese, iron, cobalt, nickel and copper.l In itsusual form the metal is an alloy of nickel and iron.

Whatever its specific composition, it is dis#` tinguished by itspronouncedmagnetic propv i ertles and especlallythe remarkably low valueof magnetomotivepfor -"2715e"quired to' produce saturation. Thesemagneticiproperties depend j-ulargely onf-thespecial heat treatmentwhiclrislgiven to :the alloy-*during its process of manufacture."-'-'A`'A' Emagnetic core modulator, made of a core; und`-with a ribbon ofythatmaterial, the wholegicoilbeing a proximatelythe size of ningen-ring,was" ound tov operatein the system'o Fig. 8,-w1'th substantially thesame eic'ienoy and to give substantially A as 'good reproduced y speechas. wlth' the'use of the conventional. I carrier currentv systems usingvacuum tubes' l and second order modulation. .Referencefis madetoa'paper by Arnold and Eline'n in the Journal of the llranklinInstitutefor May,1923,'forafdetaileddescripuonof this -alloy and its properties.

' AlthoughnFg- Slthe ptenti'als'areshown .Y i I' `impressed on themodulator in series, the'y' may equally well be impressedin parallel,the

particular arrangement of circuits depending upon the impedances of thelters outside the transmission bands, andtherefore being dev'termined'bythe laws governing the eilicient connection of circuits generally.I Ingeneral filters having low impedance outside the -v band shouldI befconnected in series. If the impedances outside the bands'are high,theyl should be connected inparallel. A- conductive instead of aninductive connection be-V tween the modulator circuit and filter FB maybe used; 'I4i'gs. 10y and 11 illustrate tir-rangements; which cnductivstonnes' modulation; and ,des instit tions sie liittd,-. antiiiliistjftte further the of lseries and. parallel connections ofcircuits. figures illustite; the case' of 'eithermodulatin 6rdemodalstjion and.

also the use. of the .same fof both .Fer this interesse, ex'ien-tymnyufge thegnse' t @deferment-designed" i one,

-. 'signin wave tmpiitude inail-sw sgml-.wave frequencygandtheaotheg.thetn. mater,-

" ing the arrangement' lfi Vi beingJ desifned'for low modulated wave'ntilde en higiiiiiiiaituted ime- .lgreqllend Modulationjis' here 'eil'mary *windingY as in theabove age). Further, the capacitive' vbe vt"ix'1creaae .-thea'liux de`ns ity .for a oad .ful 'the ecient operationofthe lnodutoligb'yfle'asonA of the'sel'e'ctive characteristic of thev-circuit referred tothe'priinaryiif seconda '.is tuned .toy resonanceAthe', carrier 1 u'enc. The 's'electivecharactenst ic t at' thisfeature..

The modulating carrier vand side band -currents, or certain of Y to flowthrough separate .coi son the core, Vas. in Figs.y 13 and 14.l i

appropriately denotedv longitudinal magnetization, since the dunes'resulting,"l`

from the two magnetomo'tive forces follow arallel, that is, the samepaths. Ity ,has n found that a. similareif'ect isl produced bywhatma becalled cross v"magnetization Iin -whic the windings are 'so arranged'that 'the two fluxes. tend to lbejno'rinaljto eachother. The lthreecircuits'v for. the

`modulating, 'oarrie'r, and 'sid'e band currents may be otherwiserelated in eitherof the ways above described.' .use of. crossmagnetization has an advantage, in a modulatoror a demodu1ator,. thatmutual indue# tion between the 'modulating and carrier cir` cuits 'maybe made substantially zero', there-y fore avoiding the necessity 'of theuse of filters in these circuits.' In*l a' combined Insta statista*itititaiiiitafwiiiafi .l

qluency characteristic, filter` Fn described, together.' with thevarying 'mutual'. e (varyingle secondar` load, by reducing thedemagnetf'" 'izing e ect of ,the mndsrygeufrenajnsy' .be' similarltheinJna even be caused modulator anddemodulator, however, only thecarrier filter vmay be omitted. The effect Thear'rangemen may ibe;varied .considera ly.. from that shown.. -Forlexample, the unmodulatedcar- -rvier maybe lntlfoducedat the receiving sta- 'tion-,f-.fin'stead''off being transmitted thereto vfrom thev transmitti i .source 12'?,vswitch' 1 5 being c osed. The carrier. may, of course, betransmittedfrom station, b means of the transmitting station by an independentmit-modulated current of a particular'fremay be transmitting Vconductor`18. If even order demo'dulati-on' isr used corres onding. even harmonicsofthe unmodulate carrier may transmitted or locally introduced at t ereceiver.

t ofi-,thas stem of Fig.. 8f

For example, the Asignal may be reproduced by second order-dc'modul'ation if ythe double carrier 2p is "'ci'rcuitQ When 1t is notimportant'to transself-inductanoe and' "of the i-.p'n-ir used. The'double'y carrier may be conveniently derived from source 14, is bythecir- 'cuit described in U. S. patent to Kendall, No. 1,446,752,issued February 27 '1923, by ning' 'switch 123, so that current fromcircuit4 v20 is supplied froni source 14 through harmonic generator GH.It may be derived from double carrier frequency stliur 124 at thereceiver, switch 126 being c ose A` substantial vdegree of secrecy maybe obtained with the system of Fig. 8 by producing lalower third orderside band, or a side bandhavingsimilar frequency characteristics, supressing the double carrier fre- '.The examples `of magnetic coilmodulatorsdescribed'above illustrate what 'might method, that is, secondorder demodulation,

HNI

llo

will give an audible frequency component represented by (2p-q)p which 1ssubstantially inverted speech. Any 'signal obtained by incidental thirdorder of modulation will be effectually masked by this inverted speech.Only by using a demoduator which gives no second order side band as. forexample, by using the magnetic demodulator of :the type shown in Fig. 8,may undistorted speech be obtained. The same princi lc lmay be appliedif other orders of modu a tion and'demodulation are used.

Multiplex signaling may be achieved will: the system of Fig. 15 by usingdifferent orders of modulation, or at least using except vfor thespecifc features of the present invention, may be similar to that ofconventional multiplex carrier current systems, thesystem is illustrateddiagrannnatically'. Referencenumerals 24 and 25 indicate twotransmitting' circuits of a multiplex carrier system.' Thecorresponding' receiving cirl cuits are indicated by 26 and 27 and 28indifrequency carrier through filter F2.

cates the line circuit. -Low frequency signals from lines 29 and 30modulate carrier c urrent derived from source 31 in modulators M1 andM2. The former produces second, the latter third, order side bands orside bands of similar frequency characteristics if produced by othermethods which are selectively transmitted by liltcrs F1 and F2. Thetransmitted side bands are selectively received by filters RF1 and RF2and demodulated by demodulators DM1 and DMB, the former being of thesecond, the latter ofthe third order type or of other types producingsimilar demodulation products. If the un modulated carrier istransmitted, it may be utilized in the usual manner in the respectivedemodulators. It, however, may be supplied toboth demodulators, or toeither, by separat-e local source 31. or amplilied `carrier fromdemodulator DM1 may be supplied to demodulator DMZ, 'through filter F2..Demodulator DM2 may, if desired,be of the second 'order type and besupplied with doull course, other orders of demodulation may be used forthe two channels disclosed. Additional channels using additional ordersmay be used. In particular, even order modulation (odd harmoniccarriers) may be used for all the channels. If the harmonic frequencycarriers are transmitted second order demodulation may be used, whichprovide-sa simple means for deriving from a'single/carrier source thevarious frequencies used in conventional multiplex harmonic systems.Since the carrier'frequencies are odd militiples of the basic carrierfrequencv they cannot produce interfering effects by intera ctien, aswould occur if even multiples were used.

The result achieved is similar to that of a multiplex harmonic systememploving the usual second order modulation. However, besides theadvantages which inliere in high order modulation as pointed out above,the svstem as a whole is characterized by a sim-- plicity which is notfound in other harmonic systems. especially on account of the avoidan'eof the use of harmonic generators.

Fig. 16 illustrates the use of a similar principle in a two-waysignaling system in which the frequencies for the two directions arefixed by a single source. The two Oneway channels may be joined to asingle line in each station by aconjugateconnection 'as shown. i Thechannel identified by'M TF1, 28, RF DM1 may be identical with thecorrespondingly labeledr channel in Fig, 15.

Similarly channel M2, TF2, 28, RF2 and I DM2 may be identical with theother chancurs in tie opposite direction. Source 81 supphesthe currentfor,l thefirstchannel nel of Fi 15, except that transmission 0c.

and demodulated`carrier for demodul'ator *I DM2 may be selected by.filter Fzand supplied to the second channel.l Obviously the two-waychannel of. Fig. .16 may be duplicated to constitute a multiplexsignaling system having allof the features of the system of Fig. 15.- l

Although, of the higherorders, the application of odd order modulationhas been emphasized as compared with' even order,

modulation. this' is only because lthe range of odd. orders ofmodulation includes the lowest of these even higher orders. Theamplitude of the `side band of the several orders in gene-ral varies'inversely' as the order. as has been explained. This means practi^allythat in general, :third order modulation is -to be preferred. A secondreason for the emphasis on odd order modulation is that this type ofmodulation can conveniently be accomplished to the exclusion of evenorder modulation, a result frey quently desired. ,The converse result,thaty is` even` order modulation -to the -exclusion of odd ordermod-ulation,is not readily accomplislied.

It is realized in view of the similar physi'al operation involved inmodulation .and demodulation, thatthere should be a single Word`toinclud'e v both or* either .of these operations. Accordingly, in theclaims the word modulation will be understood, where applicable, tocover vboth modulation' and demodulation as ordinarily used.

tically the .same frequency val-ues vas the high order sidebandsproduced'by.themethod de scribed in this specification:may'be,producedA i It is also realized that vside 'bands of lideni,

cessive steps performed in `di-iferent.modul lating devices. Forexample, side 'bands of vfrequencies Qpfl-fg .may "be produced bydoubling the carrier .frequency lin -one step i of second ordermodulation ,and then isepa.-

ratelv combining Y.the wave of `.this double carrier frequency with 'thewave of frequencies f] inthe same yor `anotherfdevice byA another stepof second order modulation. This process lis yold .in the prior Vartand' differs fundamentally froml'thisfinvention as hcreinbeforedescribed. Consequently, it is" vnot-'desired to claim.' herein asnewper se rocessof,ol"v means4 for pr ucing side andsv of ,thej formmping, mtl-'n greater than 2, vwhere theirproduction invo v devices;although -the inventive idea illustrated in Figs. and lincludes-thepromethods and Jbylother 'methods capable Vof producing" side bandsof`similar frequency produced', are -alike' in j their quency mp.'

Since a be combined with` either the original carrier fixed freor anymultiple thereof,"depending. on the l' onthe same distorting medium, andutilizingusefull band order of demodulationused, 'a word is similarlyrequired toi express' fthese concepts. Consequently,v the word multiplewill be used in the claims in a generic sense to include the single aswell as4 plural multiples unless such an interpretation .would beinconsistent with. the context." 25

What is claimed i's:

1. The method hich comprises jointly impressing 'a carrier wave and asignal wave on the same distorting medium, and utilizingr one or more'of the resultant sidebands of higher than the second-order. f 2. Thef"ethod lwhich comprises jointly y impressing acarrier wave and a signalwave resultant modulated componentscorresponding to at least one orderof modulation higher than the second. i

The method, using a distorting device which vhas yaninput-potential-output-current y l characteristic, the general equationof which includes'odd power terms, which consists in jointly limpressingwaves of diireent frequencies on said device and utilizing theresultantcombination `frequencies of higher -order than the second resultingifromsaid odd order terms. 4. The method using av distorting device whichconsists-in jointly impressing waves of different frequencies on saiddevice, combining said frequencies in a single step -to 4 produceside-bands of higher order lthan the second, and leading oi from saiddevice -and employing currents of saidv siderequencles. 5. The method ofsignaling which comprises jointly impressinga carrier wave and ,a signal'wave on the same distorting med1um,.transmitting the resultantmodulated components having higher order thany second, and demodulatingsaid components 'to reproduce the signal.

f 6.l The method'of signaling which com-v A prises jointly impressing acarrier waveand a sional-wave on a 'd istorting'deviee, transmitting atleast one side band corresponding "to at least one order ofmodulationhigher es successive separate steps of modula- 'tion ordistortion yin the sameor ldiii'erent.

however highI order modulated wave .mayy

'prises generatlng by hlgher order modulaband with a wave whosefrequency ris 'amul- ".tiple of the unmodulated carrier frequency in adistorting device, toreproduc'e the signal.

7. In a systemincluding a single distorting aling Vwhich comdevice themethod-.of si prises generatingl by higher j order modulation andtransmitting a modulated wave 'y having lthe frequencies mping, in whichfm.

and -nare integers, at least one of which has a value greater thanunity, is the unmodulatedcarrier frequency, and g is the modulatingfrequency.

8. he method of signaling which com tion a modulated Wave havingfrequencies mpi-g in which m is an integer'having a i value greater thanone, and p and g are rerespectively theunmodulated carrier andmodulating frequencies, transmitting said '-wave, and combining it atthe receiver with a wave having a frequency which is a submultiple of mto reproduce the modulating wave.

9. In a system including a single distort. ing device the method ofsignaling which comprises generating by higher 'order modul lation andtransmitting a modulated wave having the frequencies 2pig, in which pand g are respectively the unmodulat'ed carrier and modulatingfrequencies. l0. The method of signaling which `Colnprises generating amodulated wave of the third order, transmitting said wave, and

modulating it at the receiver with a carrier r current' to reproduce themodulating current.

11. The method of signaling which comprises generating modulated sidebands of the third order, transmitting said side bands, transmitting awave of the. original carrier frequency, intermodulating saidtransmitted third order side bands, and indicating the third order sideband which corresponds to .the modulating wave.v

13. The method of signaling iwhich com- `side bands and carrier wave insuch a manprises generating modulated side'bands of the third order,transmitting said side bands, intermodulating at the receiver saidtransmitted. side bands with a wave having double the4 original carrierfrequency inl such a manner as to produce second lorder modulatedv sidebands, and indicating the second ordery side 'band which corresponds tothe modulating wave. i

14. The method ofv signaling which comprises generating and transmittinga modulated wave 'having only the frequencies mpig, in Which p' and gare respectively the carrier and modulating frequencies and 'm is aninteger greater than one, and reproducing the modulated wave at thereceiver by combining said transmitted wave with a carrier wave.

15. The method of signaling which comprises generating a carrie;1 wave,generating a modulating Wave, producing odd order modulated side bandsof said carrier wave, andat the same time suppressing the'even orderside bands and even power carrier terms, suppressing the unniodulatedcarrier and transmitting the resultant waves.

16. A secrecy carrier signaling method which consists in generating andtransmit.

ting a Wave having only the frequencies 2pg and p, in which g `is thefrequency of the modulating current and p is the unmodulated carrierfrequency, which is so chosen that p-g is within the audibleirange, pro--ducing third order modulated side bands from the received currents ofsaid frequencies, and indicating that side band which corresponds to themodulating current.

1 7. A multiplex` harmonic carrier trans-l mission. system comprising,means for gen erating a carrierwave, a single modulating deviceindividual to ea'ch channel for modulating a portion of said carrierWave with one of a'plurality of signal waves to pro- 5' duce, with allof said devices, a plurality of sets of side band waves of the type mpiqin which m is different for each such set of side band waves, means fortransmitting said side band waves, and means at thereceiver forcombining said side band waves with a multiple frequency carrier currentto reproduce the respective signal currents.

18. A multiplex harmonic carrier transmission system comprising mcansfor generating acarrier wave, a single modulating device individuall toeach channel for' modulating a portion of said carrier wave lwith one ofa plurality of signal waves to produce, with all of said devices, aplurality of sets of side band Waves of the type mpi g in which m isdifferent for each such set cf side band waves means for transmittingsaid side band waves together with va carrier wave whose frequencyV isamultiple of theoriginal carrier frequency, inclu ing such frequency;means at the receiver for-modulating one of said setsof side band waveswith said multiple frequency waves to re-v produce one signal wave;means for selecting afportion of the amplified multiple fre quency wavefromthe modulated output of said modulating means, and means forreproducing other signal waves by .modulating the other side band waveswith said selected multiple frequency carrier wave.

isV

bands with some of the unmodulated carrier to the other station, meansfor modulating said side bands with the unmodulated carrier at the otherstation to reproduce the signal, means for selecting a portion of theamplified carrier resulting from demodula. tion at the other station, asingle modulating" means for intermodulating said selected carrier and asignal wave at the other station to produce side bands having a fixedfrcquency which is a multiple, greater than one of the Iixed frequencyof the wave 'receive at the other station, transmitting at least one ofthe last mentioned side bands to the iirst station, and means formodulating the side bands received at the first station with a portionof said generated carrier wave.

20. A signaling system comprising a plurality of one-way channels fordifferent transmissions and means whereby'- each channel employs 'anorder of modulation different from the others.

21. A signaling system comprising two one-way channels for differenttransmissions each employing a different order of modulation. 22. Aduplex system in'which the oppo sitely directed, channels employdifferent orders of modulation. n 4 23. A signaling system comprising,a. plurality of one-way channels, a carrier waveA ico".

is different from the corresponding xed frequency of the side bands inthelother l channels anda multiple of the frequency of vsaid carrierwave includingv said carrier (wave. f

. 24. A signaling system comprising two one-way channels and a carrierwave source,

each ofsaid'channels comprisino' a 'singlev modulating device formtermodui-atmg said carrier waves with signalwaves to produceA -sidebands the fixed frequency of which is different from that of thecorresponding side bands in the other channel and a multiple ofsaidcarrier frequency, including the frequency of the carrier.

. 25. The method of modulation which comprises impressing the modulatingand carrier waves uponla distorting medium, and

controlling the medium to have a sharply f curvedinput-voltage-output-current characteristic, whereby pronounced highorder modulation side bands are produced.

26. The methodof producing odd order lv.side by aA distorting devicewhich'con- V said y'double curvature;-

sifstsinimpressmg the carrier 'and modulat ingI waves o'na' distortingdevice, the y'characteri'stic curve between the'input andA out- -putquantities of which has a double curva- ,'even'order modulation effects,using a disopposite directions and which are symmetritorting device,which consists in impressing the carrier and modulatin'g'waves ona'distorting device whose operating characteristic has two pronouncedbends in relatively cal about a--poin-t in the curve, and operatinguniformly .over said bends inbothdirections from said point.

28."'1he ,method of producingodd order which consists in variablysaturating the ance with` modulatin f magnetic-core in accordance withimpressed alternating carrier and modulating currents,

and variably inducing alternating potentialsin 'jaccodance with saidvari-able saturation.

29,]Themethod of producing odd order modulation side bands using amagnetic core, 'a magnetizing winding, and an inductor winding, whichconsists in varying the permeability of the core by and inaccordcurrents flowing in the magnetizing winding, through maximumvalues injtheltwo directions, and thereby invariableE. M. F.s in saiVinductor Win ing.

30. The method of ymagnetic core modulation using a'magnetic core and acombined magnetizing and inductor Winding which consists in varying thepermeability of the lcoreyby and in accordance with carrier andmodulatin currents flowing through said winding, t rough maximum valuesin the two directions, and thereby correspondingly varying the impedanceof said Winding whereby odd order modulated side band currentsv arecaused to flow in said winding.

31. A magnetic core modulator comprising in confibinationa toroidalmagnetic core, a magnetizing coil therefor, means for caus- 4to amaximum ineither direction.

modulation side bands using a magnetic coreing carrier and modulatingcuirents1to flow in said coil, the values of said currents an themagnetic proportions of said core being 'i such lthat the core isperiodically substans4 .ti-ally saturated for opposite directions offlux, and an inductor circuit wherein there` is induced variable E. M.\F. 's corresponding to the variable saturation of said core.

32. `A 'magnetic core modulator .compris. ing in combination a magneticcore, means whereby carrier and modulating currentsproduceva magneticiux in said core, and4 means whereby said currents together periodicallyvary the permeability -of said core 33. The magnetic core modulatorspecified in-cl'a-im 32 in which the magnetic core Y tied in claim 32 inwhich the core is of a magnetic material comprising two elements of themagnetic group.

35. The magnet-ic core modulator "specif fied in claim 32 in which thecore is of a magnetic material comprising nickel and another member ofthe magnetic group.

36. The magnetic core modulator specified inclaim 32 in which the coreisof a magnetic material comprising nickel iron.

37. The magnetic core modulator specilied in claim 32 in which the coreis of a magnetic material comprising nickel and another element of themagnetic group, the nickel component predominating and the materialhaving a permeability higher than that of iron at low magnetizingforces.

38. The magnetic core modulator specified in claim 32 in which the coreis of a magnetic material comprising nickel and iron in which the nickelcomponent predominates and having a permeability higher than that ofiron at magnetzing forces of the order of a few tenths of a gauss.

In-witness whereof, I hereunto subscribe my name this 26 day ofDecember, A. D.

and

